1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display device in which a pixel is disposed at each intersection portion of a plurality of signal lines and a plurality of scanning lines, and a pixel electrode and a transistor are disposed at each pixel.
2. Description of Related Art
A vertical (V) lines inversion driving method and a Vertical/Horizontal (V/H) lines inversion driving method have been generally known as methods for writing a video signal to each pixel electrode in an active matrix liquid crystal display device.
As shown in FIGS. 1A and 1B, in the V-lines inversion driving method, each of the video signals having a polarity inverted for each signal line wired in parallel to a vertical scanning direction is written to each pixel. When scanning shifts from an arbitrary n-th frame to an (n+1)-th frame, the polarity of video signal in each pixel is inverted. Specifically, the polarity of video signal in each pixel is inverted each vertical scanning period. In FIGS. 1A and 1B, the symbol “+” indicates a positive polarity pixel, and the symbol “−” indicates a negative polarity pixel. In the V-lines inversion driving method, when a common potential is set to, for example, 5V, a voltage of 9V is applied to positive polarity pixels, and a voltage of 1V is applied to negative polarity pixels.
As shown in FIGS. 2A and 2B, in the H/V-lines inversion driving method, the polarity of a video signal is inverted for each signal line, and the polarity of the video signal is inverted for each scanning line. When scanning shifts from an n-th frame to an (n+1)-th frame, the polarity of the video signal in each pixel is inverted.
However, in the V-lines inversion driving method, when the potential at the signal line varies for some reason, the potential at the pixel electrode is varied due to the existence of coupling capacitance between the signal line and the pixel electrode. Moreover, the polarity of a certain pixel and the polarity of each of two pixels adjacent to the certain pixel in a horizontal scanning direction are opposite for each other. Therefore, when a rectangular complementary color window pattern is displayed at the center of a screen with a halftone color used as a background color, an amount of a potential variation at each pixel electrode differs from one pixel to another. As a result, the gradation of a halftone color luster of the window pattern differs in its right and left portions thereof as well in its upper and lower portions thereof, causing display unevenness called vertical cross talk.
In the H/V-lines inversion driving method, the polarity of the video signal is inverted each horizontal scanning period to cope with such a situation. Since the inversion of the polarity of the video signal cancels the potential variation at each pixel electrode each horizontal scanning period, the vertical cross talk can be reduced. However, the cycle for inverting the polarity of the video signal is short, and power consumption is increased.
A final screen of Windows (trade mark) adopted as an OS for many personal computers is a checkered pattern expressing black display pixel groups and halftone display pixel groups alternately as shown in FIGS. 3A and 3B. With respect to the halftone display pixels, while the number of negative polarity pixels is larger than that of positive polarity pixels in the n-th frame of FIG. 3A, the number of positive polarity pixels is larger than that of negative polarity pixels in the (n+1)-th frame of FIG. 3B. Thus, polarity deflection occurs in the halftone display pixels, and brightness differs between positive polarity pixels and negative polarity pixels. Accordingly, this deflection is prone to be visible as flicker. The number of the positive polarity pixels and the number of the negative polarity pixels in the halftone display differ from each other in each scanning line, causing polarity deflection in this direction. For this reason, horizontal cross talk may occur due to influences of potential variations at opposed electrodes formed on the surface of an opposed substrate which is disposed so as to face an array substrate where pixel electrodes, signal lines and the like are formed.
Incidentally, in the active matrix liquid crystal display device, a pixel transistor is formed for each pixel, and a liquid crystal display device using an amorphous thin film transistor (TFT) or a polycrystalline silicon TFT as the pixel transistor has been known.
In the liquid crystal display device using the amorphous silicon TFT, a tape carrier package (TCP) in which a signal line driving circuit and a scanning line driving circuit are formed on a flexible wiring substrate is used. When the TCP is connected electrically to a connection terminal of the array substrate, the signal driving circuit is connected to pixel transistors via signal lines and the scanning driving circuit is connected to pixel transistors via scanning lines on the array substrate.
In the liquid crystal display device using the amorphous silicon TFT, wirings for outputting the video signals from the TCP onto the signal lines are needed. However, since the number of the wirings becomes large accompanied in addition to the pixels being highly minute, it is difficult to secure sufficient pitches between the wirings.
On the other hand, in the liquid crystal display device using the polycrystalline silicon TFT, the driving performance of the pixel transistor is high, and hence the signal line driving circuit and the scanning line driving circuit can be formed integrally with each other on the array substrate in the same process as that used in manufacturing the pixel transistor. In this case, part of the signal line driving circuit, for example, a digital-to-analog converter, is provided in the form of a TCP on the outside of the array substrate.
In the liquid crystal display device using the polycrystalline silicon TFT, when compared with that using the amorphous silicon TFT, the number of the wirings for connecting the TCP and the array substrate can be reduced greatly and the liquid crystal display device can be made low cost by reducing the number of external connection components. On the other hand, in the liquid crystal display device using the polycrystalline silicon TFT, the length of the wiring laid on the array substrate becomes longer in accordance with larger size of the array substrate, and video signals are deteriorated, so that display unevenness may occur.